Leprosy is a chronic granulomatous disease of the peripheral nerves and superfi-cial tissues, particularly the nasal mucosa. Disease ranges from slowly resolving anesthetic skin lesions to the disfiguring facial lesions responsible for the social stigma and ostracism of the individuals with leprosy (lepers).
The exact mode of transmission is unknown but appears to be by generation of small droplets from the nasal secretions from cases of lepromatous leprosy. Traumatic inocula-tion through minor skin lesions or tattoos is also possible. The central reservoir is infected humans. The incubation period as estimated from clinical observations is generally 2 to 7 years but sometimes up to four decades. Very rarely, cases develop in nonendemic areas without known case contacts. The infectivity of M. leprae is low. Most new cases have had prolonged close contact with an infected individual. Biting insects may also be involved. Although virtually absent from North America and Europe, there are still an estimated 10 million infected persons in Asia, Africa, and Latin America. Immigration into Western countries from areas where the disease occurs has increased the numbers of cases seen.
M. leprae is an obligate intracellular parasite that must multiply in host cells to persist. Inhumans the preferred cells are macrophages and Schwann cells. PGL-1 and LAM have been implicated in the ability to survive and multiply in these cells. The organism may in-vade peripheral sensory nerves, resulting in patchy anesthesia. Few M. leprae are seen in tuberculoid lesions, which are granulomatous with extensive epithelioid cells, giant cells, and lymphocytic infiltration. In lepromatous multibacillary leprosy, CMI is deficient, and growth of M. leprae is, thus, relatively unimpeded. Histologically, lesions show dense in-filtration with leprosy bacilli, and large numbers may reach the bloodstream.
Immunity to M. leprae is CMI mediated. The range of disease correlates with DTH respon-siveness to lepromin, a skin test antigen derived from leprous tissue similar to tuberculin.
Tuberculoid cases have minimal disease and positive skin tests. Lepromatous cases have progressive disease and negative skin tests. Other tests of CMI response toM. lepraecor- relate in the same way.